1. Field of the Invention
The invention relates to a cathode catalyst for a fuel cell and a membrane-electrode assembly. More particularly, the invention relates to a cathode catalyst having activity and selectivity for a reduction reaction of an oxidant and a membrane-electrode assembly.
2. Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and a fuel such as hydrogen, or a hydrocarbon-based material such as methanol, ethanol, natural gas, and the like.
Such a fuel cell is a clean energy source that can replace fossil fuels. It includes a stack composed of unit cells and produces various ranges of power output. Since it has a four to ten times higher energy density than a small lithium battery, it has been utilized as a small portable power source.
Representative exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell that uses methanol as a fuel.
The polymer electrolyte fuel cell has an advantage of high energy density and high power, but it also has problems in the need to carefully handle hydrogen gas and the requirement of accessory facilities, such as a fuel reforming processor, for reforming methane or methanol, natural gas and the like in order to produce hydrogen as the fuel gas.
On the contrary, a direct oxidation fuel cell has a lower energy density than that of the gas-type fuel cell, but has the advantages of easy handling of the liquid-type fuel, a low operation temperature, and no need for additional fuel reforming processors. Therefore, it has been acknowledged as an appropriate system for a portable power source for small and common electrical equipment.
In the above-mentioned fuel cell system, the stack that generates electricity substantially includes several to many unit cells stacked adjacent to one another, and each unit cell is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly is composed of an anode (also referred to as a “fuel electrode” or an “oxidation electrode”) and a cathode (also referred to as an “air electrode” or a “reduction electrode”) that are separated by a polymer electrolyte membrane.
A fuel is supplied to an anode and adsorbed on catalysts of the anode and is oxidized to produce protons and electrons. The electrons are transferred to a cathode via an out-circuit, and the protons are also transferred to the cathode through the polymer electrolyte membrane. In addition, an oxidant is supplied to the cathode, and then the oxidant, protons, and electrons are reacted on catalysts of the cathode to produce electricity along with water.